This invention relates to the biological treatment of wastewater, and more particularly to an improved form of rotating biological contactor.
Rotating biological contactors are used in secondary biological wastewater treatment processes. The contactors provide surfaces for the growth of a biomass which has the ability to absorb, adsorb, coagulate and oxidize undesirable organic constituents of the wastewater and to change them into unobjectionable forms of matter. The contactors are typically rotated partially submerged in wastewater in a treatment tank so that the surfaces are alternately exposed to the wastewater and to oxygen in the overlying atmosphere. A film of wastewater is carried into the air and trickles down the surfaces of the contactor while absorbing oxygen from the air. Organisms in the biomass remove dissolved oxygen and organic materials from the film of wastewater and unused dissolved oxygen in the wastewater film is mixed with the contents of the mixed liquor in the tank.
Initially the rotating biological contactors were simple flat discs spaced along a support shaft. Originally formed of sheet metal, the flat discs have more recently been formed of a foam plastic material (see British Pat. No. 935,162 to Hartmann, published Aug. 28, 1963). Discs offer a limited surface area in relation to the volume which they occupy, and other approaches have been employed to increase the surface area in relation to the volume of the envelope of the contactor. One approach has been to build up the contactor from sheets of thermoformed plastic which are joined together along the length of the axis of the shaft and define a series of passageways through which the wastewater flows (see U.S. Pat. No. 3,827,559 issued Aug. 6, 1974 to Gass, et al.) The passageways define large surface areas which are contacted by the wastewater and support the growth of the biomass. Another approach is to wind a formed sheet of thermoplastic in a spiral about a central shaft. The formed sheet can be provided with a series of cups or other protuberances which hold the layers of the convolute apart and also increase the surface area (see U.S. Pat. No. 4,115,268, issued Sept. 19, 1978 to Thissen).
The contactor is subjected to considerable load. Although the biomass is relatively thin on any surface (typically 1 to 4 mm thick), in the aggregate the weight of the biomass is considerable and particularly when it is saturated with a wastewater as it must be in order to function properly. The drag through the water as the contactor is rotated is a further source of loading. Finally, there may be some buoyant force particularly when a foamed disc contactor is employed. In an effort to overcome the loading problems to which the contactors are subjected, metal frameworks have been used. A framework typically includes radial struts eminating from the shaft and connected adjacent their outer ends by generally tangential bars. The framework also includes tie rods that extend the length of the contactor parallel with the axis of the shaft and connected to the radial struts. The purpose is to better transmit the load on the contactor to the shaft and the torque of the shaft to the contactor. These metal frameworks must be specially coated or formed of stainless steel in order to resist the very corrosive environment in which the contactor will operate.
Where a supporting framework is not employed, stress failures can occur particularly in the area where the contactor material is joined to the shaft.
The present invention provides a rotating biological contactor construction which does away with the need for a supporting framework while providing a strong contactor structure and an excellent joining of the contactor to the supporting shaft.